Electrodes which are used to record biopotentials from the surface of the skin generally require the use of a conductive liquid or solid gel to provide a continuous conductive path between the recording surface (i.e., the skin) and the electrode sensing element. Conductive gels contain a salt (KCl or NaCl) in order to achieve electrical current flow. The preferred gel is one with a high salt content, since such a gel produces a better conductor than that obtained when using a gel with a low salt content. In addition, the use of a high salt content gel typically requires less skin abrasion at the time of application to reduce the impedance of the skin-electrode interface after subsequent electrode application.
For ease of use, it is desirable to apply the conductive liquid or solid gel at the point of manufacture, creating a xe2x80x9cpre-gelledxe2x80x9d electrode. U.S. Pat. No. 4,559,950 issued to Vaughn and U.S. Pat. No. 5,309,909 issued to Gadsby describe two such electrodes. Use of such electrodes saves the user the step of manually applying the gel to the electrode at the time of electrode application and speeds the application process considerably. Thus, the ideal electrode would be one pre-gelled with high salt content conductive gel. Such an electrode would minimize application time by reducing the amount of skin xe2x80x9cpreppingxe2x80x9d (abrasion) required by low salt content gels and eliminating the step of dispensing the gel onto the electrode surface.
Numerous prior art references exist that show that one can make a pregelled electrode by chloriding the surface of a silver substrate to create a Ag/AgCl electrode element with a stable half cell potential. Often, a silver-plated plastic substrate is used instead of solid silver, for cost reasons. An electrolytic gel may then be applied at the time of manufacture to create a pre-gelled electrode.
It is common in the art to construct single-piece sensors which incorporate multiple electrode elements on a single substrate. Such sensors have the advantages of low cost, ease of use, and precise positioning of the electrode elements. A common method of construction, as described in U.S. Pat. No. 5,337,748 issued to McAdams, utilizes a flexible circuit, created by printing a circuit on a plastic substrate using conductive ink. The conductive ink makes up the electrode sensing element and provides an electrical connection between the individual electrode elements and a cable connector, which facilitates connection to a data acquisition system. The conductive ink generally consists of flakes of silver (Ag) in a liquid binder. U.S. Pat. No. 4,852,572 issued to Nakahashi describes a pregelled, multiple electrode sensor constructed by printing a single layer of conductive ink on a non-woven cloth substrate. It is not possible, however, to pre-gel sensors constructed using conductive inks with liquid conductive gels because the salt content of the liquid gel quickly reacts with the Ag flakes in the ink and renders the circuit non-conductive. Such a process would lead to early sensor failure and a reduced shelf life. For this reason, sensors constructed using conductive inks are pre-gelled using a cured solid hydrogel with a low salt concentration. The use of a low salt content gel slows the rate at which the salt content of the gel corrodes the Ag element and therefore extends product shelf life. The impedance of the skin-electrode interface is generally higher than that which could be achieved with a high salt content gel, and the resultant electrical signal is much noisier. In addition, vigorous skin prepping is required to lower the impedance to an acceptable level, due to the limited hydrating properties of a solid gel.
Various prior art constructions exist which use multiple layers of conductive inks for low fidelity applications, such as the acquisition of resting EKG signals. One such construction is the TCP-3208 conductive coated polyester manufactured by Tolas Healthcare Packaging which includes a layer of conductive carbon material underneath a layer of conductive Ag/AgCl. The main purpose of this construction is to minimize the amount of silver (Ag) on a circuit trace, thus reducing the manufacturing cost. Such a construction generally makes use of solid hydrogel as the ionic interfacing material.
Another prior art construction, which is described in U.S. Pat. No. 5,337,748 issued to McAdams includes a single layer of Ag or Ag/AgCl ink on a flexible substrate such as vinyl or Melinex to make an electrode. Again, a solid hydrogel or a low concentration of salt in the liquid gel must be used in order to obtain an acceptable shelf life. Carrier in U.S. Pat. No. 5,352,315, teaches the use of a single conductive ink layer of either Ag/AgCl or a homogenous mixture of Ag/AgCl and carbon inks printed on a nonconductive backing layer.
U.S. Pat. No. 4,787,390 issued to Takata teaches the use of snap and eyelet type construction, though in this case the snap and eyelet is simply used to make mechanical contact between different components of the electrode and not to provide a pressure-sealed gel isolation function. U.S. Pat. No. 4,444,194 issued to Burcham and U.S. Pat. No. 4,617,935 issued to Cartmell also teach the use of a snap and eyelet construction, but only for the purpose of physically connecting electrode components.
It is therefore a principal object of the present invention to provide an electrophysiological electrode that utilizes a flexible circuit construction while allowing for the use of high salt content liquid electrolytic gels.
Another object of the present invention is to provide an electrophysiological electrode that contains a single interfacing contact to an electrophysiological monitor or other data acquisition system.
It is a further object of the present invention to provide an electrophysiological electrode with pre-gelled electrodes which provide low impedance while reducing the need for skin preparation.
This invention is an electrophysiological electrode that includes multiple layers of materials to isolate liquid electrolytic gels from the conductive inks on the flexible circuit of the electrode substrate. Such an electrode has a much longer shelf life under normal storage conditions than other electrodes of such construction with high salt content liquid electrolytic gel, and is able to maintain acceptable impedance upon its eventual use.